A silicon nitride sintered body obtained by molding and heating/sintering a silicon nitride powder is excellent in the mechanical strength, corrosion resistance, thermal shock strength, thermal conductivity, electrical insulation property, etc., and therefore, being used, for example, as a wear-resistant member such as a cutting tip and a ball bearing, a high-temperature structural member such as an automotive engine component, and a circuit substrate. In the application to a circuit substrate, etc., a silicon nitride sintered body satisfying, among others, both high thermal conductivity and high mechanical strength at levels not less than before is demanded.
As such a silicon nitride sintered body, it is considered to be preferable to use, as the sintering aid, a magnesium oxide and a rare earth oxide, which are unlikely to raise the mechanical strength but likely to provide a sintered body having high thermal conductivity, in place of an aluminum oxide and a rare earth oxide, which facilitate obtaining a sintered body having high mechanical strength, and in addition, to use a silicon nitride powder having a reduced impurity content or a silicon nitride powder having an adjusted oxygen content, etc., as a raw material.
For example, Patent Document 1 discloses a silicon nitride sintered body obtained by sintering a silicon nitride powder having a β proportion of 93%, an Al content of 150 ppm and an oxygen amount of 0.9 mass %. Specifically, in Examples thereof, a silicon nitride sintered body having a thermal conductivity of 95 to 118 W/m·K and a three-point bending strength of 660 to 900 MPa is disclosed.
Patent Document 2 discloses a silicon nitride sintered body obtained by sintering a silicon nitride powder containing 1.1 mass % of oxygen, 0.10 mass % of an impurity cation and 97% of α-phase silicon nitride and having an average particle diameter of 0.55 μm. Specifically, in Examples thereof, a silicon nitride sintered body having a thermal conductivity of 50 to 130 W/m·K and a three-point bending strength of 600 to 850 MPa is disclosed.
Patent Document 3 discloses a silicon nitride sintered body obtained by sintering a silicon nitride powder having a specific surface area of 5.6 to 28.9 m2/g, specific proportions of internal oxygen and surface oxygen, and a specific particle size distribution, the silicon nitride powder being obtained by firing an amorphous Si—N(—H)-based compound having a small oxygen content ratio relative to the specific surface area at 1,450 to 1,650° C. by heating at a temperature rising rate of 12 to 100° C./min in the temperature range of 1,000 to 1,400° C. while flowing the compound in a continuous firing furnace. Specifically, in Examples thereof, a silicon nitride sintered body having a thermal conductivity of 130 to 142 W/m·K and a three-point bending strength of 605 to 660 MPa is disclosed.
Patent Document 4 discloses a technique for producing a silicon nitride sintered body having a thermal conductivity of 80 W/m·K or more, a three-point bending strength at room temperature of 600 MPa or more, and a fracture toughness of 5 MPa·m1/2 by firing a silicon nitride powder having a particle size distribution with d10, d50 and d100 of 0.5 to 0.8 μm, 2.5 to 4.5 μm, and 7.5 to 10.0 μm, respectively, and having an oxygen content of 0.01 to 0.5 wt %.